Within the field of watch-making, a conventional architecture is used to make movements, which are provided with striking mechanisms, such as alarms or minute repeaters. In such embodiments, the gong or gongs used are each formed by a metal wire, which is generally circular in shape and placed in a parallel plane to the watch dial. The metal wire of each gong is generally arranged around the movement, in the watch frame and above a plate on which the various parts of the movement are mounted. One end or several ends of each gong are fixed, for example by soldering, to a gong-carrier integral with the plate, for example, which may be common to all of the gongs. The other end of each gong may generally be free.
The watch striking mechanism includes at least one member for activating the gong, which may be a hammer activated at predetermined times. In this case, the vibration of each gong is generated by the impact of the corresponding hammer on the gong, in particular, in proximity to the gong-carrier. Each hammer makes a partial rotation in the plane of the gong(s) so as to strike the corresponding gong and vibrate it in the plane thereof. Part of the gong vibration is also transmitted to the plate by the gong-carrier.
The mechanical impact between the hammer and the gong of a conventional striking mechanism is difficult to control. The same is true for optimization of acoustic efficiency, which is greatly limited within the audible frequency range, particularly within the range of frequencies between 1 kHz and 4 kHz, but also between 4 kHz and 20 kHz. This is due to the fact that the mechanical impact of the hammer against the gong is of very short duration and most of the energy is transmitted at high frequency vibration modes above 4 kHz. Likewise, the mechanical shock activates almost all of the gong vibration modes, but without allowing any selection of the activated modes. The duration of impact of the hammer against the gong generally cannot be increased by altering the geometry, inertia and material of the parts involved, without also causing a marked decrease in the impact energy. Due to the internal damping and acoustic radiation, the sound cannot be extended without repeating the mechanical shocks. Further, mechanical shocks, particularly the impacts of the hammer against the gong, may lead to spurious noise, especially in the case of double impact, and cause wear of the gong, which constitute drawbacks.
FR Patent No 1 214 428 discloses a striking device for a clock. This striking device includes, in particular, a rotatably mounted hammer, driven by means of an electro-magnet in the direction of a bell, to generate a sound during the mechanical impact of the hammer against the bell. As mentioned hereinbefore, any mechanical impact of the hammer against the bell may also cause spurious noise, which is a drawback.
CH Patent No. 634 455G, which discloses a watch provided with an electroacoustic vibrator, may also be cited. The vibrator includes a vibrating membrane mounted on a shoulder of a support secured to the case, and a coil mounted on a magnetic core, and arranged at a distance underneath the vibrating membrane. An annular magnet is also placed around the coil as part of the magnetic circuit with the magnetic core. A plate made of soft material is secured to the membrane to close the magnetic circuit without any mechanical contact. When a current passes through the coil, a force for activating the membrane is applied to the plate to generate an acoustic signal without any mechanical contact. One drawback of this type of electroacoustic vibrator is that an electrical control device is required to power the coil in order to vibrate said membrane, which requires considerable electric power consumption.
FR Patent Application No. 2 236 223 A1 discloses an acoustic sound generator for a wristwatch. This generator includes a ferromagnetic pin fixed to an inner part of the watch crystal, and an electro-magnet for vibrating said pin. The electro-magnet includes, in particular, a coil arranged in a magnetic circuit provided with permanent magnets. When the coil is powered with electricity, the pin starts to vibrate to generate an acoustic sound. One drawback of this generator is that the coil has to be powered with an alternating current to activate the vibration of said pin and the resonance of the watch crystal, which requires significant electric power consumption.
EP Patent Application No. 0 963 033 A1 discloses a device for generating an acoustic signal in a watch. This device includes a vibrating strip, which is provided with a permanent magnet or a moving steel part, and an activation member, which includes an electro-magnet opposite the permanent magnet of the strip. When the electro-magnet is activated, the vibrating strip starts to vibrate to generate an acoustic signal. As in the preceding documents, the electro-magnet has to be powered by an alternating current in order to vibrate the vibrating strip, using significant electrical power, which is a drawback.